Mystery Sandstone Spheres Follow Up (Part 2)

This is a continuation of my immediately preceding post.

Where the mantle is absent, such as bare spots on ridge tops and gullies, especially on their sloping slides, the spheres commonly cover much of the exposed surface. The above view is of the side of deeply incised canyon with abundant exposed spheres, most of which are about 10 mm in diameter. Staff is 2 m in length.

Close-up of spheres from previous photo. Geological Society of America scale, 10 cm intervals on left side of scale.

Freshly broken layer shows no spheres on fresh surface (along right side slab) compared to the older surface with many spheres attached. This photo is very important because it shows that the  spheres form on exposed surfaces and are apparently the result of weathering (i.e., formed after the sediments became rock).

Spheres of various sizes on vertical surface. Within a given layer, the spheres are fairly uniform in size. In incised washes, the spheres occur in multiple layers, which can be interspersed and/or bounded by sphere-free layers. The beds that contain them can be over a meter in thickness and can be completely covered by the spheres on exposed surfaces.

                                              

The spheres are not armored-mud balls (interior is mud and the exterior is coated by an assortment of angular particles of many sizes), which form by rolling along the floor of a desert stream. Armored-mud balls do occur in the same area as the spheres but are comparatively uncommon; one is shown on the right. On the left, is an unusually large and liberated sand sphere with its characteristic smooth surface.

ADDITIONAL OBSERVATIONS:

The spheres are not concretions. Unlike concretions, the spheres have a uniform composition all the way through (see the original post), without concentric shells, and they do not leave concave depressions behind where they were formed. Concretions are hard solid masses that form slowly via chemical changes induced by groundwater percolating through the sediments before they become a sedimentary rock.

The presence of spheres in a road cut along Interstate 15 (see index map in preceding post) indicates that the spheres do not require geological time to form. Interstate 15 was completed in 1964, so the road cut containing the spheres was made sometime just prior to that construction phase. Because the spheres require surface exposure to form, this puts an upper limit on the formation time at the road cut to less than 55 or 60 years. They might well require a much shorter time to form than that.  

Mystery Sandstone Spheres Follow Up (Part 1)

This is a follow-up of my post on “Mystery Sand Spheres,” issued February 11, 2018. 

David Liggett, who provided the initial reconnaissance information used in the February post, recently made another field trip to the Mojave Desert, California and made in-depth, very revealing observations about these mysterious structures.  The comments given below are from him, and the photos were taken by him.

AREAL DISTRIBUTION: (as currently known)

The green line in this Google Earth image shows the approximate boundary of the area that I covered.  (This was a reconnaissance trip, and I didn’t thoroughly cover all of this area.)  The area west of the red line contains abundant lake beds, and I did not recognize any spheres in association with the lake deposits.  I would expect that the spheres continue to the northeast and northwest beyond what I have shown here.  Because the spheres are in an I-15 road cut I would expect them to continue to the southeast as well.   (I also found an isolated, small occurrence of the spheres in Dunn Wash so they could expand across Dunn Wash to the west also.)  I did not look anywhere south of I-15.

The spheres were found at an elevation range of 510 to 585 meters.  There doesn’t appear to be any genetic relationship to the late Pleistocene Lake Manix shorelines.  Both Miller et al. (2014) and Reheis et al. (2014) (see below) have mapped the green-enclosed area, but there appears to be no mention of anything like the sand spheres in their papers.  However, this is a marginal area, away from their focus of interest: Miller et al. was mainly interested in Fort Irwin to the north, and Reheis et al. was focused on Lake Manix to the south.

Overview of the environment where the spheres occur. The foreground in mostly devoid of the overlying fanglomerate mantle, and the "bare areas" are nearly covered with thousands and thousands of small spheres.

Abundant spheres on vertical surface overlain by beds containing fewer spheres which are, in turn, overlain by rock mantle. Some of the layers contain no spheres.

Sphere-bearing layers commonly form where where the rocky mantle of fanglomerate (pebbles and cobbles of angular igneous an metamorphic rocks) cover is thin, usually on a sloping surface. 

The spheres shown here are mostly ones before they become detached from the host bed. Car key for scale. The spheres are composed of the same material as their host beds; mainly a sandstone with some clay fraction. They are not made of grus per se; probably an arkose. They are weakly to moderately cemented between your fingers. The degree of cementation seems to an important factor in the sphere formation. Not enough cement and the spheres will not form, or if they do forms they won't be able to withstand further weathering; that is to say they will not last long after formation. Too much cementation and the spheres will not separate from the surrounding sandstone.

Spheres in various stages of development on a smooth surface (bedding plane?).

The spheres in the studied area range from 5 to 40 mm in diameter, but most are about 10 mm in diameter. The sphere size is generally uniform within any given layer, but their size can vary widely from one layer to another.

The references cited above are:

Miller, D.M. et al. 2014. Generalized surficial geologic map of the Fort Irwin area, San Bernardo County, California, chap. B of Beusch, D.C., ed., Geology and geophysics applied to groundwater hydrology at Fort Irwin, California. U.S. Geological Survey Open-File Report 2013–2014, 11 pp., scale 1:100,000, http://dx.doi.org/10.3133/ofr20131024B
ISSN 2331-1258 (online)

Reheis, M.C., et al. 2015. Surficial geology and stratigraphy of Pleistocene Lake Manix, San Bernardino County, California: U.S. Geological Survey Scientific Investigations Map 3312, 46 pp., 2 sheets, scale 1:24,000, https://dx.doi.org/1.3133/sim3312.
ISSN 2329-132X (online)